1. Field of the Invention
In one respect, this invention relates generally to a method for the preparation of 2,6-dimethyldecalin from a 12 carbon atom-containing dicyclic naphthenic isomer thereof. In another respect, this invention relates generally to a method for the preparation of a mixture of 2,6-dimethyldecalin and a 12 carbon atom-containing dicyclic naphthenic isomer thereof. More particularly this invention concerns the aforesaid methods employing in each a catalyst system comprising a hydrogen fluoride solution of tantalum pentafluoride and/or niobium pentafluoride.
2. Description of the Prior Art
Dimethyldecalins (dimethyldecahydronaphthalenes) are useful intermediates for the preparation of various intermediates containing functional groups. Particularly, dimethyldecalins can be dehydrogenated to form dimethylnaphthalenes which can be oxidized under controlled conditions, as exemplified by Saffer et al., U.S. Pat. No. 2,833,816, to form the corresponding dicarboxylic acids. A naphthalene dicarboxylic acid in which the carboxyl groups are located at the 2.6 positions is a highly desirable article of commerce, particularly in that it can be used for making polyester-type resins which have outstanding properties in various applications.
A difficulty in the commercial preparation of 2,6-naphthalene dicarboxylic acid lies in finding a suitable source of 2,6-dimethylnaphthalene. This compound occurs in coal tar and cracked petroleum fractions of appropriate boiling range but only in low concentrations since it is associated with the various other dimethylnaphthalene isomers, monocyclic aromatics, and non-aromatic hydrocarbons. A concentrate of the dimethylnaphthalene isomers can be obtained by solvent extraction of the fraction with an aromatic-selective solvent such as furfural, but the 2,6-isomer is still only a minor constituent of the concentrate. Attempts to isomerize the other isomers, particularly those in which both methyl groups are positioned on the same ring, to produce only the 2,6-isomer have not been successful.
Consequently, the preparation of 2,6-dimethyldecalin for use as an intermediate in the preparation of 2,6-dimethylnaphthalene has received considerable attention. For example, Schneider, U.S. Pat. No. 3,243,469 discloses a method for the preparation of 2,6-dimethyldecalins by isomerization of any dicyclic naphthene of 12 carbon atoms utilizing a catalyst system of aluminum halide and hydrogen halide at a temperature of -10.degree. C. to 60.degree. C. The isomerization product is an equilibrium mixture of dimethyldecalins including 2,6-dimethyldecalin. After removal of the catalyst, 2,6-dimethyldecalin can be separated from the equilibrium mixture by fractional crystallization at a temperature below -10.degree. C. The other isomers can be recycled to the isomerization step for further equilibration, to ultimately convert essentially all of the original dicyclic naphthenes being converted to 2,6-dimethyldecalin.
Schneider, U.S. Pat. No. 3,346,656, discloses a method for preparing dimethyldecalins from naphthenes of 6 carbon atoms in which a naphthene of 6 carbon atoms or a mixture of such naphthenes is contacted at a temperature in the range of -20.degree. C. to 80.degree. C. with a preformed liquid catalyst complex obtained by reacting a paraffin hydrocarbon having at least 8 carbon atoms per molecule with AlCl.sub.3 -HCl or AlBr.sub.3 -HBr. Under these conditions, the naphthene of 6 carbon atoms dimerizes to form a dicyclic naphthene of 12 carbon atoms which isomerizes to an equilibrium mixture of dimethyldecalins. Schneider, U.S. Pat. No. 3,219,718, discloses a method for the preparation of decalins by the rearrangement of uncondensed dicyclic naphthenes having 2 cyclohexyl rings utilizing an aluminum halide-hydrogen halide catalyst. This patent discloses that any uncondensed dicyclic naphthene having 12-20 carbon atoms and 2 cyclohexyl rings in the presence of such catalyst at a temperature in the range of -20.degree. C. to 70.degree. C. will rearrange to form decalins having the same empirical formula as the dicyclic naphthene. The decalins formed when relatively long reaction times are used are an equilibrium mixture of isomers having the same number of carbon atoms per molecule as the dicyclic naphthene used as the starting material.
Suld et al., U.S. Pat. No. 3,200,161 disclose another method by which any dicyclic naphthene containing 12 carbon atoms can be isomerized to 2,6-dimethyldecalin. The naphthene rings of the starting material can be either condensed or noncondensed, and any alkyl substituent or substituents can be included that will result in the naphthene having 12 carbon atoms. Disclosed means for obtaining such starting materials include separation from suitable petroleum fractions, hydrogenation of coal tar fractions, and dimerization of methylcyclopentane and/or cyclohexane. In the disclosed method, the dicyclic naphthene is contacted with a catalyst containing hydrogen fluoride, a promoter, and an initiator at a temperature in the range of -10.degree. C. to -60.degree. C. The promoter can be either boron trifluoride or an antimony pentafluoride. The initiator can be any olefin, alcohol, ether, or alkyl halide containing not more than 5 carbon atoms. Within a short time interval after isomerization commences, 2,6-dimethyldecalin begins to precipitate. Thereafter, isomerization and precipitation occur simultaneously and continue until over 50 weight percent of the starting material has been recovered as solid 2,6 -dimethyldecalin.
A method disclosed in abandoned patent application Ser. No. 69,798 filed Nov. 17, 1960 has also been reported in the aforesaid Suld et al., U.S. Pat. No. 3,200,161, according to which any dicyclic naphthene containing 12 carbon atoms can be isomerized to an equilibrium mixture of dimethyldecalins in which 2,6-dimethyldecalin occurs in relatively high proportion. The method comprises contacting the dicyclic naphthene with an aluminum bromide-hydrogen bromide catalyst at a temperature in the range of from about 10.degree. C. to 60.degree. C. 2,6-Dimethyldecalin can be separated from the resulting equilibrium mixture by cooling the mixture to about -20.degree. C. to -40.degree. C., for within this temperature range the 2,6-isomer selectively crystallizes, the other isomers remaining in the liquid phase.
Bushick et al., U.S. Pat. No. 3,509,223, disclose a method for dimerizing monocyclic naphthenes in the presence of a catalyst system and a suitable hydrogen acceptor. Any naphthene containing 6 carbon atoms is suitable for use as a charge stock in such method. The catalyst system employed for the dimerization consists of hydrogen fluoride, boron trifluoride, and a hydride acceptor-chain initiator. The hydride acceptor-initiator is an organic compound containing less than 6 carbon atoms, which is generally an olefin or an alkyl halide, although alcohols and ethers are also functional. In accordance with such method, the naphthene or a mixture of such naphthenes is contacted at a temperature in the range of -20.degree. C. to 80.degree. C. with the catalyst system. Under these conditions, the naphthene dimerizes to form a dicyclic napthene containing 12 carbon atoms, which then isomerizes to form an equilibrium mixture of dimethyldecalins. The equilibrium mixtures contain approximately 30 percent each of 2,6- and 2,7-dimethyldecalins.
A major problem in each of the aforesaid prior art methods is that the relative yield of 2,6-dimethyldecalin is low and/or that the method is a multi-step method and/or that a promoter and/or initiator is required for the reaction. The initiators or promoters conventionally used in the aforesaid prior art methods generally have the side effect of acting as a catalyst poison and hence of shortening useful catalyst life. Such problems can be overcome by the method of this invention which, unlike the aforesaid prior art methods, employs a catalyst system of tantalum or niobium pentafluoride in hydrogen fluoride.
Hydrocarbon conversion processes involving the use of catalysts comprising tantalum or niobium pentafluoride in hydrogen fluoride have been extensively described in the prior art. For example, Lien et al., U.S. Pat. Nos. 2,683,763 and 2,683,764 disclose that the combination of hydrogen fluoride with either tantalum pentafluoride or niobium pentafluoride are powerful catalysts for isomerization, alkylation, cracking and other reactions. Oelderik et al., U.S. Pat. No. 3,201,494 disclose that niobium pentafluoride or tantalum pentafluoride in combination with hydrofluoric acid can be employed for the isomerization of hexane. McCaulay et al., U.S. Pat. No. 4,214,116 and U.S. patent application Ser. No. 47,059 filed on June 11, 1979 and now U.S. Pat. No. 4,246,094, disclose improved processes for isomerizing and improving the octane rating of hydrocarbons in the naphtha boiling range.
Siskin et al., U.S. Pat. No. 3,852,184, disclose a process for upgrading reformer feedstocks containing alkylcyclopentanes by isomerizing the feedstock components in the presence of a catalyst mixture containing a metal halide, such as tantalum and/or niobium pentafluoride, and a protonic acid, such as hydrogen fluoride, and preferably in the presence of hydrogen, at a temperature in the range of -30.degree. C. to 125.degree. C., so that the alkylcyclopentanes are converted to the corresponding cyclohexane isomers. Siskin et al., U.S. Pat. No. 3,948,761 disclose a method for isomerizing acyclic and alicyclic aliphatic hydrocarbons by contacting the same with hydrogen in the presence of a difficultly reducible metal halide, such as tantalum pentafluoride, niobium pentafluoride, or their mixtures, in combination with at least a molar equivalent of hydrogen halide. Typically, acyclic hydrocarbons having at least 4 carbon atoms, that is, straight chain or branched chain paraffins having from 4 to 10 carbon atoms, are converted to branched materials having higher octane ratings. Additionally, alicyclic hydrocarbons having at least about 5 carbon atoms, such as cyclohexane, can be converted to isomers thereof by the same method. This hydroisomerization is performed at temperatures in the range of from about 0.degree. C. to about 150.degree. C. The patentees of U.S. Pat. Nos. 3,852,184 and 3,948,761 do not disclose dimerization of methylcyclopentane or cyclohexane to form 12 carbon atom-containing dicyclic naphthenes such as dimethyldecalins. In fact, analysis of the products of the specific examples in these patents indicated a net decrease in the concentration of dicyclic naphthenes from that in the reactants. Furthermore, the patentees disclose that the presence of sulfur compounds or aromatics does not adversely effect the catalyst. In each of the specific examples, substantial quantities of aromatics and/or sulfur compounds are present. The examples of U.S. Pat. No. 3,948,761 also employ feeds in which the concentration of methylcyclopentane and/or cyclohexane is too low, according to the present invention, for dimerization to occur.
However, thus far a catalyst system comprising tantalum pentafluoride or niobium pentafluoride in hydrogen fluoride has not been employed in a method for preparing 2,6-dimethyldecalin from its 12 carbon atom-containing dicyclic naphthenic isomers or for preparing mixtures of 2,6-dimethyldecalin and its 12 carbon atom-containing dicyclic naphthenic isomers as dimerization products.